How the eye works

A helpful way of understanding how the eye works is to compare it with a camera. The cornea is the clear window at the front of the eye, analogous to the front lens in a camera. It provides some protection from trauma and focuses incoming light rays. The crystalline lens is deeper inside the eye and acts like the main lens in a camera. It focuses incoming light rays further, bringing them to a point of focus on the retina, the thin layer inside the back of the eye which acts like a sensor in a digital camera. Light falling on the retina triggers electrical signals. These signals are sent to the brain via the optic nerve, which behaves like an electrical cable. The image we appreciate is formed by processing in the visual area of the brain.

Refractive error

In order to achieve clear vision, incoming light-rays must be focused sharply on the retina. People whose eyes are incapable of providing a sharp focus without the need for spectacles or contact lenses, have what is called a refractive error. There are three basic types of refractive error as outlined below. The absence of refractive error is termed emmetropia.

Short-sightedness (myopia)

Short-sightedness is when spectacles or contact lenses are required to provide clear distance vision (e.g. driving and watching TV). This is usually due to the eye being too long, the cornea being too steep, or some combination of the two.

Long-sightedness (hyperopia)

Long-sightedness is when spectacles or contact lenses are required to provide clear near vision (e.g. reading or using a computer). This is usually due to the eye being too short, the cornea being too flat, or some combination of the two. Often, people who are long-sighted also begin to lose their clear distance vision once they reach their forties.

Astigmatism

For practical reasons the normal cornea can be thought of as being shaped like a soccer ball rather than a rugby ball. A person with astigmatism has a cornea which is shaped more like a rugby ball. This results in light rays being focused at two points near the retina, rather than at a single point on the retina. The result is blurred vision. Low degrees of astigmatism are quite common and usually do not require correction. However, people with higher levels of astigmatism may find spectacles or contact lenses provide them with much clearer vision for certain activities. Astigmatism is commonly found in association with either short- or long-sightedness.

Presbyopia

Presbyopia is a form of refractive error which results in the need for reading glasses. It is due to natural ageing processes in the eyes and affects everybody around their forties and fifties. People who are short-sighted may find the onset of presbyopia is delayed, whereas people with long-sightedness may find it develops earlier.

Laser vision correction

There are some great lifestyle benefits to being less dependent on spectacles or contact lenses.

Laser vision correction is the method of altering the focusing properties of the eye (refractive error) by reshaping the cornea using the excimer laser. Miniscule amounts of corneal tissue can be removed using one of two principal techniques: PRK or LASIK. Both eyes are usually treated on the same day to avoid problems associated with a difference in focus between the two.

Laser vision correction performed by ophthalmologists at Eye Associates is carried out using the latest technology, including the femtosecond laser for creation of the LASIK flap (so-called ‘bladeless LASIK’).

Not everybody is suitable for laser vision correction. At Eye Associates a comprehensive assessment can be performed, including a topographical analysis of the cornea (Pentacam), to determine which technique of laser vision correction will best suit each individual.

Corneal transplantation

o The cornea

The cornea is the clear window at the front of the eye. It is approximately 0.5 mm thick and 12 mm across. Behind it is the fluid-filled anterior chamber, which contains the coloured iris. As mentioned above, the cornea is like the lens of a camera – any opacity or distortion results in a poorly focused image on the retina.

The cornea has 3 main layers:

The thin surface skin (called the epithelium)

The thick central layer (called the stroma), and

The single layer of cells on the back surface (called the cornealendothelium). These are attached to the corneal stroma by a fine layer called Descemet’s membrane.

All of these layers must be healthy and smooth for the cornea to work as a clear window.

Conditions causing scarring of the cornea, such as chemical injuries, trauma, and infections, may result in impaired vision due to alteration of the shape of the cornea or poor transmission of light to the retina.

Corneal transplantation is a means of reducing visual impairment due to irreversible diseases of the cornea and has been performed for over 100 years. However, recent developments in surgical technique have made corneal transplantation available to a broader range of patients with less trauma to the eye and a more rapid visual recovery.

o Types of corneal transplantation

Penetrating keratoplasty (PK)

In this oldest form of corneal transplantation, the full thickness of the diseased cornea is replaced with that of a healthy donor cornea. The new graft is sewn into place with very fine sutures, which are usually not removed until at least one year later. Most people require spectacles or a contact lens to obtain their best vision in the operated eye. Clearest vision may not be achieved until one or two years after the surgery. Anti-inflammatory eye drops are required indefinitely in the eye, to prevent the graft undergoing rejection by the recipient’s immune system.

Deep anterior lamellar keratoplasty (DALK)

This type of corneal transplant is used in people who have scarring or disease only involving the superficial portion of the cornea. In this technique, the patient’s corneal endothelium and deepest corneal stroma is left intact, but the corneal epithelium and the bulk of the stroma are replaced with healthy donor tissue. The advantages of this method include greater stability of the wound and the ability to remove the sutures earlier than in PK. In some cases, DALK may be preferred to PK in suitable patients with keratoconus.

Endothelial keratoplasty (DSEK or DSAEK)

This type of corneal transplant is used for conditions caused by failure of the corneal endothelium, such as Fuchs’ corneal dystrophy.

The cells of the corneal endothelium constantly pump fluid out of the cornea into the anterior chamber in order to prevent the cornea becoming waterlogged. If these cells stop working properly and the thickness of the cornea gets above 0.6 mm it starts to become foggy (called corneal decompensation). Once it gets over 0.8 mm thick the cornea develops blisters on the surface (epithelium) leading to pain as well as foggy vision (a condition called bullous keratopathy).

DSEK and DSAEK are acronyms denoting surgery in which the patient’s endothelium is stripped off and replaced with the endothelium from a healthy donor cornea. The advantages of this technique include less trauma to the eye and faster visual rehabilitation compared with PK.

o All forms of corneal transplantation carry a risk of immunological rejection. People are warned to seek attention immediately if the transplanted eye becomes red, sore or the vision becomes blurry. Prompt treatment with intensive anti-inflammatory drops can avert irreversible failure of the transplant.

o Assessment of suitability for corneal transplantation can be performed by ophthalmologists at Eye Associates.

Collagen cross-linking

People with keratoconus have a condition in which the corneas gradually become more misshapen with time. This results in the need for progressively stronger spectacles, often rigid contact lenses, and in some cases a corneal transplant.

Keratoconus usually develops during the teens and can progress throughout the twenties and thirties. Thereafter the cornea becomes naturally stiffer through ageing and most cases of keratoconus become relatively stable by a person’s forties.

Research over the last 10 years has demonstrated that a method of artificially stiffening the cornea using application of vitamin B2 and exposure to ultraviolet light, called collagen cross-linking, can halt progression of the disease. This technique is recommended for people who have documented progression of keratoconus, in order to reduce the likelihood of ultimately requiring a corneal transplant.

Assessment of suitability for collagen cross-linking is provided by ophthalmologists at Eye Associates.

Other corneal treatments offered by ophthalmologists at Eye Associates include:

Intrastromal corneal ring segments (Kerarings) – some people with keratoconus may be suitable for these.

Phototherapeutic keratectomy (PTK) – people with recurrent corneal erosions or certain types of corneal scarring may benefit from this type of laser treatment.

Diagnostic technology

Corneal tomography (Pentacam)

Corneal tomography is a rapid method of accurately measuring the shape of the cornea. It uses a rotating camera to determine the elevation of both the front and back surfaces of the cornea, reconstructing a virtual 3-dimensional model which can be analysed in great detail. It is a useful tool for assessing corneal diseases and plays a critical role in determining suitability for people wishing to undergo laser vision correction.

Corneal pachymetry

Corneal pachymetry is a fast and reliable method of measuring the thickness of the central cornea using ultrasound. It is useful in monitoring how swollen the cornea is in conditions where its water pumping mechanism is not working normally, such as Fuchs’ corneal dystrophy. It is also helpful in conditions where the cornea becomes thinner than usual, such as keratoconus.